1. Field of the Invention
The present invention relates to a telecentric image-forming optical system through which a principal ray passes in a direction parallel to the optical axis of the system, and more particularly, to a telecentric image-forming optical system in which an image size is large compared to the distance between the object and the image, and the magnification of the system is one or approximately one.
2. Description of the Prior Art
In known telecentric image-forming optical systems, the secondary focal point of a first lens group having a positive focal length and disposed in front of a lens disposed at the center of the optical system coincides with the principal focal point of a second lens group which also has a positive focal length and which is disposed behind the center lens. In this arrangement, when an object (or a real image formed by an optical system disposed in front of the first lens group) is placed in the vicinity of the principal focal point of the first lens group, a real image is formed in the vicinity of the secondary focal point of the second lens group. The magnification of the real image corresponds to the ratio of the focal lengths of the lens groups.
If the intervals between the principal points of the lenses are ignored, the distance between the object and the image is approximately twice as great as the sum of the focal lengths of the lens groups.
In such a system, it is difficult to create a wide angle view. Therefore, if a large image size is required, the optical system itself must be large, which causes an apparatus which incorporates the optical system to be undesirably large.
Apparatuses in which the optical system is incorporated include an image reader for reading an original document by a solid image sensor (for example, CCD), and exposing apparatus for printing mask patterns of an IC and an LSI on a semiconductor wafer, a scanning recording apparatus such as a laser plotter, a laser printer and the like. A telecentric image-forming optical system is useful in these apparatuses because the magnification of such a system does not vary if the object distance is changed. Further, using a telecentric image-forming optical system prevents degradation of resolution and reduction of the intensity of marginal rays.